The present invention relates to an accurate control device for a transmission for an automotive vehicle and to a method practiced by said device, and, more particularly, relates to a control device, incorporating an electrically actuated hydraulic fluid pressure control system and an electronic control computer which controls said electrically actuated hydraulic fluid pressure control system, said control device accurately controlling selective supply of hydraulic fluid pressures to a hydraulically controlled transmission so as to accurately set it to any one of its various speed stages, and to a control method for said hydraulically controlled transmission practiced by said device.
Automatic transmissions for automotive vehicles which include gear transmission mechanisms including several fluid pressure activated friction engaging mechanisms such as multi plate clutches and multi plate brakes are well known in various forms. Such an automatic transmission is usually conventionally controlled by a hydraulic fluid pressure control system, which selectively controls supply of hydraulic fluid pressure to the friction engaging mechanisms, according to decisions that said hydraulic fluid pressure control system makes as to what speed stage of the gear transmission mechanism should be currently engaged, in view of and based upon the current values of various operational parameters of the vehicle, such as the road speed of the vehicle, the load upon an internal combustion engine of the vehicle, and the like.
In more detail, it is well known and conventional for such a hydraulic fluid pressure control system to receive supply of a throttle hydraulic fluid pressure which is produced as the output of a throttle hydraulic fluid pressure control valve, and which varies according to the amount of depression of an accelerator pedal of the vehicle incorporating the transmission, which is taken as indicative of the load on the internal combustion engine of the vehicle, and also to receive supply of a governor hydraulic fluid pressure which is produced as the output of a governor hydraulic fluid pressure control valve, and which is varied according to the road speed of the vehicle. Such a conventional hydraulic fluid pressure control system, in order to decide upon a speed stage of the gear transmission mechanism which is to be engaged, evaluates various relations between the magnitudes of the throttle hydraulic fluid pressure and the governor hydraulic fluid pressure, by supplying these pressures to various shift valves so as to act upon the valve elements thereof in opposition, according to various equilibrium relationships, and, based upon the movement of these valve elements, supplies of activating fluid pressure to the various friction engaging mechanisms such as the aforementioned clutches and brakes are made, in order to engage the appropriate speed stage which has thus been decided upon.
Nowadays, however, with the rapid progress which is being attained in the field of electronic control systems, various arrangements have been proposed, in which electronic control circuits make control decisions as to what speed stage of the gear transmission mechanism should be currently engaged, in view of and based upon the current values of various operational parameters of the vehicle, such as the road speed of the vehicle, the load upon an internal combustion engine of the vehicle, and the like. The current values of these operational parameters are sensed by sensors which dispatch signals to the electronic control system via A/D converters and the like. In such arrangements, electric signals are sent by such an electronic control system to electric to hydraulic pressure conversion devices such as solenoid valves, and these electric to hydraulic pressure conversion devices perform the actual switching of the aforesaid activating hydraulic fluid pressures to the friction engaging mechanisms.
Many conventional such electronic control systems for providing actuating hydraulic fluid pressures for engaging the friction engaging mechanisms of gear transmission mechanisms with four forward speed stages are essentially similar, in their basic control logic, to the above described conventional hydraulic fluid pressure transmission control systems, in having a first and second speed switching valve, which controls the switching between the first speed stage and the second speed stage, a second and third speed switching valve, which controls the switching between the second speed stage and the third speed stage, and a third and fourth speed switching valve, which controls the switching between the third speed stage and the fourth speed stage. In other words, the only difference is that the switching controls of these transmission switching valves are performed by electrically actuated means, such as solenoid valve devices or the like, based upon control judgements made by an electronic circuit, instead of according to the above described type of set of equilibrium relations between throttle hydraulic fluid pressure and governor hydraulic fluid pressure, etc., as in conventional hydraulic fluid pressure transmission control systems. Thus, according to this basic structure, in the case of a gear transmission mechanism which has four forward speed stages, as outlined above three transmission switching valves are required, and also three solenoid valves are required for controlling these three transmission switching valves.
As opposed to the above outlined basic structure for an electric transmission control system, it has been previously remarked upon that, since it is possible to achieve switching between two alternatives by using one switched valve, i.e. by using one combination of a transmission valve and an electric actuator such as a solenoid, then, by using a combination of two such switched valves, it should be possible to switch between four different combinations, according to the four possible combinations of switching available from two binary switching devices. This is based upon the fundamental concept that 2.times.2=4. In this case, certainly the structure becomes simplified, because the number of shift valves is reduced to the absolute minimum of two. Such a form of transmission control system, for example, is disclosed in Japanese Patent Publication No. 5128/73. A more developed form of such transmission control system is disclosed in Japanese Patent Application No. 69110/80 which was filed previously to the filing of the parent Japanese patent application No. 114062/81 of the present application of which priority is being claimed in the present application, and for which prior art concept also it is known to the present inventor that U.S. patent application Ser. No. 06/263,261 has been filed previous to the filing of the present application claiming the priority of said previous Japanese application, said previously applied Japanese and U.S. patent applications relating to said prior art concept being invented by different inventors than the present application; and the present inventor hereby desires to acknowledge his debt to this previous proposal, and to incorporate the subject matter of that previous U.S. patent application by reference into the present application, by way of background prior art.
Further, an automatic transmission conventionally includes a fluid torque converter, which provides a fluid coupling between the engine of the vehicle and the gear transmission mechanism, thus eliminating the need for any clutch system for the drive train of the vehicle, allowing smooth shifting of the transmission between its various speed stages while said transmission is transmitting rotational power while the vehicle is moving, and also allowing for the vehicle to be stationary while the engine is turning at a low rotational speed at or close to the idling speed without the engine stalling, as well as providing torque multiplication by fluid flow in a per se well known way when the vehicle is being accelerated at relatively low speed and relatively low engine rotational speed. Many such torque converters are of course presently well known. Generally, such a torque converter comprises: a housing of a generally toroidal shape, on the inside of which there are formed a series of vanes which constitute a pump impeller, and fixed to a power input shaft; a turbine member mounted within the housing as fixed to a power output shaft; and a stator member mounted within the housing via a one way brake on a fixed member. The housing of such a torque converter is kept filled with hydraulic fluid, which is pumped thereinto and is also drained therefrom, and in a per se well known way the pump impeller, the stator member, and the turbine member cooperate, when the housing of the torque converter is thus filled with hydraullic fluid, to define a toroidal hydraulic fluid flow circulation system, circulation of hydraulic fluid around which in the general circulation fashion of a ring is arranged to transfer torque in a conventional manner between the pump impeller and the turbine member of the torque converter.
This supply of hydraulic fluid for filling the torque converter is typically provided to the inside of the housing thereof via a first channel defined along or beside the central rotational axis thereof--in more detail, via a hole in one of the shafts passing along said central rotational axis or through a space defined between two concentric ones of such shafts; and the draining of hydraulic fluid from the torque converter is also typically performed in a similar manner, through a second such channel. The supply of hydraulic fluid is provided, generally in the prior art, from a torque converter hydraulic fluid pressure regulation valve which supplies a supply of hydraulic fluid at a regulated torque converter hydraulic fluid pressure, which is generally rather lower than the line hydraulic fluid pressure, to the torque converter.
Further, it has become more and more common nowadays for a torque converter to be provided with a lock up clutch, i.e. a mechanical clutch which, when actuated, mechanically couples together the pump impeller and the turbine member of the torque converter with regard to their rotation, so that the above mentioned hydraulic torque transmission between the pump impeller and the turbine member no longer occurs or is relevant.
It is well known and conventional for such a lock up clutch to be engaged or disengaged according to the directions of supply and draining of the torque converter hydraulic fluid pressure to and from the interior of the housing of the torque converter. In other words, when the torque converter hydraulic fluid pressure mentioned above is being supplied to one channel which leads to the interior of the torque converter housing, and is being released from another channel, then it is arranged that the lock up clutch is engaged; and when the torque converter hydraulic fluid pressure is being supplied to said other channel, and is being drained from said one channel, then it is arranged that the lock up clutch is disengaged. Thus the supply of torque converter hydraulic fluid pressure to the torque converter from the torque converter pressure regulation valve is used for two purposes: to fill the torque converter with hydraulic fluid; and to selectively engage and disengage the lock up clutch, according to the direction of said supply.
The selective engagement of this lock up clutch, in the prior art of hydraulic fluid pressure control systems for transmissions, is typically performed by a control device such as a hydraulic fluid pressure control device incorporated in the hydraulic fluid pressure control system which controls the engagement of the various gear speed stages of the gear transmission mechanism, according to the operational conditions of the vehicle to which the torque converter incorporating this lock up clutch is fitted. In more detail, generally such a lock up clutch is desirably engaged when the torque converter is required to transmit rotary power at a fairly high rotational speed, at which time the torque conversion function of the torque converter is not required. In such a case, if the lock up clutch is not engaged, then, although the torque converter at this time provides a substantially direct power transmission function between its pump impeller and its turbine member, nevertheless a small amount, such as a few percent, of slippage between the pump impeller and the turbine member will inevitably occur, and this will waste a substantial amount of energy because of the useless churning of hydraulic fluid within the torque converter, and also will cause undesirable heating up of the hydraulic fluid contained within the torque converter. On the other hand, such a lock up clutch is desirable, of course, disengaged when the road speed of the automobile is low, or when the rotational speed of the internal combustion engine thereof, i.e. the rotational speed of the pump impeller of the torque converter, is so low as to be close to idling rotational speed, in order to utilize the buffering action of the torque converter at these times, as well as the torque multiplication function thereof. Thus, such a lock up clutch has been engaged by the above mentioned prior art type of hydraulic fluid pressure control device, typically, when and only when the vehicle incorporating the torque converter is being driven at high road speed with the gear transmission mechanism in its highest gear speed stage, with the internal combustion engine of the vehicle thus operating at fairly high rotational speed, in which circumstances the actual hydraulic torque conversion function of the torque converter is not in fact particularly required. The provision of such a lock up clutch is effective for increasing fuel economy of the vehicle, especially when running on the open road such as on an expressway.
A well known prior art construction for such a hydraulic fluid pressure control device for controlling a lock up clutch has comprised a lock up clutch control valve, which is switched between two positions, and which, when in its first switched position, switches said torque converter hydraulic fluid pressure mentioned above so as to supply it to said one channel which leads to the interior of the torque converter housing, and drains said other channel, so as to engage said lock up clutch, and which, when in its second switched position, switches said torque converter hydraulic fluid pressure mentioned above so as to supply it to said other channel, and drains said one channel, so as to disengage said lock up clutch, said lock up clutch control valve being switched to said first switched position thereof by supply of a control hydraulic fluid pressure, and, when said control hydraulic fluid pressure is not supplied, being switched to said second switched position thereof by some biasing force. Thus, when said control hydraulic fluid pressure is provided to said lock up clutch control valve, then said lock up clutch control valve is switched to its said first switched position, and accordingly the lock up clutch is engaged; and, when said control hydraulic fluid pressure is not provided to said lock up clutch control valve, then said lock up clutch control valve is switched to its said second switched position by the biasing force, and accordingly the lock up clutch is disengaged. In such an above mentioned prior art type of hydraulic fluid pressure control device, this control hydraulic fluid pressure has been provided from a hydraulic fluid pressure control system, which decides when the lock up clutch should be engaged, and which is incorporated in the automatic transmission hydraulic fluid pressure control system as a whole.
Nowadays, with the increased emphasis which is being put upon fuel economy of automotive vehicles due to rising fuel costs and greater public attention being paid to economic and pollution problems, the advantages secured by the engagement of such a lock up clutch are more and more important. In this connection, it would be beneficial for the lock up clutch to be engaged during the engagement of other speed stages of the transmission than the highest speed stage only as has heretofore been practiced, and to be engaged for a greater proportion of the time during the engagement of said highest speed stage, i.e. over a greater range of operational conditions of the vehicle. However, such increase of the utilization of the lock up clutch runs into the problem that it is very important for the lock up clutch to be disengaged at the time of shifting of the transmission between its various speed stages. This is because the shifting of the gear transmission mechanism of the automatic transmission between its speed stages, for example between its highest speed stage and its next to highest speed stage, will cause a severe transmission shock, if the lock up clutch is engaged at the time of shifting, whether the direction of the shift is upwards or downwards. In other words, the torque shock cushioning effect of the torque converter is very important when the gear transmission mechanism of the automatic transmission changes its speed stage. If the transmission changes speed stage with the lock up clutch still engaged, this can cause various undesirable effects, such as shortening the operational life of the friction engaging mechanisms of said transmission as well as the various gears and other parts thereof, and perhaps causing premature failure of the automatic transmission as a whole, as well as damaging the drivability of the vehicle and perhaps even causing a dangerous accident.
To a certain extent this problem has even occurred in the prior art when the operational range of engagement of the lock up clutch has been set in such a high speed range, i.e. above such a high predetermined road speed value, that the transmission should always be definitely set to its highest speed stage during lock up clutch engagement, because in exceptional circumstances the gear transmission mechanism of the automatic transmission may be forced to shift from its highest speed stage to a lower speed stage, even at a vehicle road speed higher than the predetermined road speed value; for example, when the driver of the vehicle forces such a shift, by moving the manual transmission shift lever of the vehicle from the "D" range to the "3" range. In such a case, if the lock up clutch is kept engaged, a severe transmission shock is liable to occur.
A possible solution for these problems that might be conceived of could be for the above mentioned hydraulic fluid pressure control system which decides when the lock up clutch should be engaged to disengage the lock up clutch during changing of transmission speed stage. However, it has been determined by the present inventor during researches relative to the present invention, as will be explained in more detail later, that the timing of the engagement and the disengagement of the lock up clutch during change of the transmission speed stage, if the lock up clutch is to be engaged both in the earlier speed stage and in the later speed stage to which the transmission is to be shifted from said earlier speed stage, is very delicate and critical, whether in fact the shift of speed stage is an upshift or a downshift. If the timing is not exactly right for the engagement and disengagement of the lock up clutch during change of the transmission speed stage, then danger exists either of the occurrence of unacceptably great variations in the output shaft torque of the automatic transmission, which are liable to be attended with the ill effects detailed above, or of the occurrence of unacceptably great variations in the engine rotational speed, i.e. of engine racing, which can cause premature excessive wear to, or even catastrophic failure of, the vehicle engine. In fact, the delicacy of timing required for such control of the lock up clutch during change of the transmission speed stage cannot practically be obtained with the above mentioned hydraulic form of lock up clutch control, because: firstly, slight manufacturing variations between various hydraulic transmission control systems, due to manufacturing tolerances, cause unacceptably great variations in timing, due to the nature of hydraulic fluid pressure control circuits; secondly, alterations in the viscosity of the hydraulic fluid in the automatic transmission, due to changes in its temperature or to its aging or to dirt particles suspended in said hydraulic fluid or the like, again cause unacceptably great variations in timing; and, thirdly, determining the variations required in the timings of engagement and disengagement of the lock up clutch, in view of the various possible combinations of upshifting and downshifting between the various speed stages of the transmission which will occur, and in view of the current values of vehicle operating parameters such as vehicle speed, engine load, and so on that must be taken account of, is quite beyond the computational capacity of a hydraulic fluid pressure control system, which is only an analog computational capacity and must be achieved with relatively few valves, passages, accumulators, and the like.
Other problems that have occurred with regard to the operation and control of such a lock up clutch relate to the operation of the braking system of the vehicle and to the operation of the engine throttle of the vehicle. In more detail, when the braking system of the vehicle is applied, if the lock up clutch remains engaged at this time there is a risk of the engine stalling; and, further, when the throttle of the vehicle is suddenly opened from the closed condition, then if the lock up clutch is engaged at this time there is a risk of a substantial torque shock being transmitted through the drive train of the vehicle.